Created Friday 30 July 2021
When copper is etched, the traces do not have a rectangular cross-section but rather typically have a trapezoidal shape. If the bottom width of the trace is unchanged, the characteristic impedance will increase when considering the etchback effect (this is due to the increased parasitic inductance and reduced parasitic capacitance associated with the shape change). In practice, we can define the shape of the trapezoid using a parameter called "Etch Angle"; which is the angle between the base of the trace and the side wall. In a rectangular trace, this etch angle would be perfectly 90°. In the absence of any other description, a good number to use for the etch angle is 70°.
Some software defines the top and bottom width of the trace rather than the etch angle. The trace thickness is also considered when determining these parameters. A simple method of determining the top width of a trace (wtop), when the bottom width (wbot), thickness (h) and etch angle (α) are known is shown below:
If the top and bottom are known the etch angle can be worked out by reversing this equation.
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